This invention relates to the field of aerodynamics, and more particularly to improvements in rotatable airfoils such as propellers, helicopter rotors, and wind powered devices for converting wind energy into electrical or mechanical power. All such devices use blades, which are rotatably mounted and have aerodynamic airfoil shapes designed according to well-known a conventional rules using aerodynamic formulas known to those skilled in the art.
The same laws of aerodynamics applying to rotary airfoils also apply to aircraft wings. The conventional formulas utilize the terminology appropriate to the aerodynamic forces on an aircraft wing, as follows:
Lift=CLxcfx81/2V2S
and
Drag=CDxcfx81/2V2S
where CL is the coefficient of lift, CD is the coefficient of drag, xcfx81 is air density, V is the air velocity, and S is the wing area. A line from the leading edge to the trailing edge of the airfoil is the xe2x80x9cchordxe2x80x9d of the airfoil, and an angle xcex1 between the chord and the direction of airflow relative to the airfoil is termed the angle of attack. CL depends mainly on the angle of attack. CL values range from negative to about 4.5, but are usually from 0.3 to 1.5. Conventional wings stall (lose lift) above an xcex1 of about 15xc2x0. CD usually ranges from 0.004 to 20, and is composed of frictional drag due to air passing over the surface of the airfoil plus other drag forces produced by separation of airflow at the top of a wing at high angles of attack and air circulating lengthwise over the wing.
It is evident that lift and drag differ only in the coefficients CL and CD in the above formulas. The coefficient of drag, CD at medium speeds (below 0.8 of the speed of sound) is due mainly to frictional drag, which increases as velocity squared. The coefficient of lift, CL increases linearly; therefore, the lower the angle of attack (xcex1) the larger will be the L/D. The highest efficiency of a rotary airfoil when the airfoil is rotated by an energy source depends on attaining the highest lift to drag ratio possible.
The design considerations are different when the rotary airfoil is receiving energy from the wind and operating a wind powered device. The basic aerodynamic formulas of an airfoil are same, except that the angle of attack xcex1 is selected to produce the maximum lift. The blade is mounted to account for the velocity of the wind relative to the rotary movement of the airfoil in a direction perpendicular to the wind velocity. Here, the object is to produce the maximum lift, which is translated to torque on the shaft of the wind powered device. Although the drag is also great, it is accommodated by the structure holding the rotating blade.
Lift or thrust is known to increase as the square of the air velocity according to the above conventional formulas. If conventional air velocity across the top of an airfoil were increased, the resulting increased imbalance in air pressure would greatly increase the lift. Suggestions have been made in the prior art for increasing the velocity over an aircraft wing for this purpose. U.S. Pat. No. 6,138,954 issued Oct. 31, 2000 to Gaunt proposes retractable slats angled above the leading edge of aircraft wings so as to form a funnel over each wing to provide increase air speed over the wing and increase lift.
U.S. Pat. No. 5,772,155 issued Jun. 30, 1998 to Nowak proposes retractable delta flaps deployed above the wing to delay flow separation on the back of the wing at increased angles of attack.
U.S. Pat. No. 1,787,321 issued Dec. 30, 1930 to Orr proposes a pair of complementary airfoils located on either side of the leading edge of an aircraft wing to form a Venturi opening in proximity to the leading edge of the wing funneling air over both the top and bottom of the wing for the alleged purpose of increasing the lifting effect during the forward propulsion of the aircraft.
Leading edge slots (known as xe2x80x9cslatsxe2x80x9d) are well-known in the prior art of aircraft wings. These comprise auxiliary members forming a contoured slot through the airfoil with an opening on the pressure side below the leading edge and exit on the suction side above and beyond the leading edge. Slats are formed by rigidly attaching a curved sheet of metal or a small auxiliary airfoil to the leading edge of the wing to form a Venturi-shaped slot. The slat prevents the breakdown in the flow over the upper surface of the wing and extends the working range of the angle of attack. The primary purpose of the slat is to prevent detachment of the airflow over the suction side of the wing.
Slats have been applied for a similar purpose to rotary airfoils, specifically propellers. U.S. Pat. No. 4,840,540 issued Jun. 20, 1989 to Kallergis, and British Patent Number 460,513 dated Apr. 4, 1936 in the name of Fairey Aviation Company Limited add slats to aircraft propellers in order to suppress flow separations and reduce noise. The propeller slats create a Venturi with opening on the pressure side of the airfoil section and exit at a higher velocity on the suction side of the airfoil section. The propeller slats of the aforementioned patents do not extend the full length of the blade, since they are intended to function only along a portion of the blade length.
Accordingly, one object of the present invention is to provide an improved rotary airfoil system providing increased lift.
Another object of the invention is to provide an improved rotary airfoil system in which the lift to drag ratio is maximized for propellers and the like.
Another object of the invention is to provide an improved rotary airfoil system for producing increased lift in a wind powered device.
Briefly stated, the invention comprises a funneled rotary foil comprising a rotatably mounted hub having an axis of rotation, a plurality of circumferentially spaced blades mounted on the hub so as to extend in a generally radial direction from the hub to a blade tip so as to be rotatable in a plane of rotation about the axis, each blade having a cross-section defining an airfoil with a suction side, a leading edge, a trailing edge, and defining a chord extending therebetween, the airfoil increasing gradually in thickness from the leading edge to a point of maximum thickness and thereafter decreasing in thickness, a fin attached to the blade tip and adapted to block radial flow from the blade tip, and a funnel strip mounted on each of the blades uniformly spaced from the suction side thereof and extending in a generally radial direction along the full length of the blade, the funnel strip having an inlet edge defining a funnel inlet area with the airfoil leading edge and having an outlet edge spaced from the suction side at approximately the point of maximum thickness to define a funnel outlet area, the funnel plate being oriented with respect to the chord so as to scoop in and increase the velocity of air flowing over the suction side of the airfoil, and dimensioned such that the ratio of inlet area to outlet area lies between and including the range of 2:1 to 20:1. Funnel angles are specified according to whether the funneled rotary foil is functioning as a wind-driving device or as a wind-driven device.